Understand the occurrence of pitting corrosion of pipeline carbon steel under cathodic polarization

Pitting corrosion has been observed to occur on pipeline steels that are under cathodic protection (CP). In this work, the square wave polarization (SWP) technique and localized electrochemical impedance spectroscopy measurements were combined to investigate the occurrence of pitting corrosion on a cathodically polarized X70 line pipe steel in a near-neutral pH solution. It was found that corrosion pits could generate on the steel under CP when it encounters a polarization fluctuation. A potential field is generated locally at defects on the steel to result in an electrochemical state conversion, i.e., the cathodic polarized state changing instantly to an anodic polarization. It was proposed that, for pipelines under CP, the generation of pits is highly possible due to the frequent fluctuations of the CP current/potential.

► Demonstrate the possibility to occur pitting corrosion on a cathodically polarized steel. ► The pitting initiation is due to the fluctuation of cathodic potential. ► Local anodic dissolution could occur at surface defects due to an electrochemical state conversion. ► Corrosion pits would generate on pipeline that experiences CP fluctuations.